Friendly Fungi - Mycorrhiza
Why plants need phosphate is something that's answered really poorly in most gardening books. You usually find something lame about it being necessary to promote rooting and flowering - especially as the best that can be said about deficiency is that the plants grow slowly without it and that they may show purple colouration in older leaves in some species.
Here's the Biochemistry.
Phosphate is what the cells use for money.
There's a magic molecule called adenosine which as well as forming half of one of the DNA nucleotide pairs is intimately involved in most of the major biochemical pathways. Talking simply in lego terms the Adenosine can have atttached to it one, two or three phosphate groupings in a chain (known as AMP, ADP and ATP respectively or to give them their full names adenosine mono, di and tri phosphate respectively). Every time the cell banks some energy from a reaction by breaking down a bigger molecule like glucose into smaller bits like carbon dioxide and water it turns an ADP into an ATP by sticking another lego brick on and every time it needs to use some to turn a building block into a glucose molecule for example it spends one and an ATP is busted down again from sergeant to corporal.
Since pretty much every chemical reaction in the cell uses this type of double entry book-keeping - no phosphate = no metabolism = no plant. That's what it does. I don't think most people who write gardening books - even "scientific" ones seem to know this.
Phosphate generally is pretty tightly bound to the soil particles and the plant often needs a bit of help to get it off. That's where Mycorrhiza come in.
Mycorrhiza - often called friendly fungi - form a symbiosis with the roots of the plant. Some types form a sheath round the outside, others actually penetrate the root. Fungi don't photosynthesise so they need to get sugars from somewhere. Having tapped into the plant they are able to help themselves to some of the goodies that the plant is producing. However symbiosis is a 2 way street.
In exchange the fungi provide the plant with phosphate because basically the fungi seem to find it easier to get it out of the soil particles than the plants do. Most plants benefit from this association although the cabbage tribe and heathers (including blueberries do not). It seems that plants in general grow better and that you can expect a 10-15% improvement in yield from adding some Mycorrhiza spores to seedlings, trees and bushes as you plant them out. The going rate is about 20 spores for a seedling and 60 per litre for shrubs, trees and hanging baskets.
We've started using themselves in the nursery -400 vines got a dusting as they were potted out before Christmas. I'll keep you posted how they do.
We thought it would be a good idea to make this available to you in an easy to use form, so we've prepared two formulations of a product which we've called RootBooster. You can buy it through our Amazon shop and there's a garden strength version and an allotment version. A large tub of the allotment formula should do a 10 rod allotment for a year.
There's some more information here